1. Field of the Invention
This invention relates to a multi-mode radio frequency (RF) receiver architecture.
2. Discussion of the Known Art
The ability to distinguish reliably between friendly and hostile approaching aircraft, is extremely important. So-called identification-friend-from-foe (IFF) systems, e.g., the Mark X IFF system, employ a single interrogation frequency (1,030 MHz) and a single reply frequency (1,090 MHz) with a standard reply code. Three coded interrogations, designated Modes 1, 2 and 3 can be selected, wherein each interrogation has a pulse spacing of 3, 5 or 8 μsec corresponding in order to the selected mode.
An airborne transponder replies with a single pulse for Modes 1 and 3, and a pulse-pair with 16 μsec spacing for Mode 2. A coder unit enables variable coded replies to be generated providing a Special Identity Feature (SIF), thus allowing a ground controller quickly to determine a particular aircraft's designation or function. Mark X SIF/IFF systems have been used in most military aircraft since 1959, and the system also serves as an aid to civil air traffic control as part of the Air Traffic Control Radar Beacon System (ATCRBS). Transponders used on civil aircraft use Mode 3 IFF, with the Mode designation “3/A” to connote common military/civil usage.
Present “Mark XII” systems operate in Modes 1, 2, 3/A, 4 and C, all of which use defined patterns of pulse amplitude modulation (PAM). Also specified are Mode S that uses phase shift keying (PSK) modulation, and Mode 5 sing spread spectrum modulation over a relatively wide bandwidth (typically 16 MHz) that is greater than the data rate.
As mentioned, current IFF systems call for transponder receivers to operate at a single center frequency of 1,030 MHz. Since 1981, a Joint Tactical Information Distribution System (JTIDS), also known as the Multi-Functional Information Distribution System (MIDS), provides military forces with both communications and navigation functions for deployment on aircraft and ships. MIDS operates on 51 frequencies in a 960 to 1215 MHz band. Because MIDS signals employ spread spectrum modulation (frequency hopping and phase coding) techniques, they represent potential interfering signals to IFF transponders having receivers tuned at 1,030 MHz, notwithstanding the two closest MIDS frequencies are each 22 MHz away from 1,030 MHz, i.e., at 1,008 MHz and at 1,052 MHz.
Specifically, since emitted MIDS signals use spread spectrum modulation, broadband noise generated by a MIDS transmitter may be within the passband of an IFF transponder receiver. To meet this problem, transponder receivers have incorporated both narrow band RF preselectors and narrow band intermediate frequency (IF) channels to achieve reliable interrogation signal detection in the presence of MIDS signal interference. But certain performance parameters such as pulse fidelity, phase distortion, inter-symbol interference and receiver group delay have bene compromised, however. And, while a narrow band receiver front end may allow Mark XII and Mode S interrogating signal waveforms to be detected reliably while rejecting MIDS signal interference, such an approach is incompatible with Mode 5 IFF spread spectrum applications which require a much larger receiver bandwidth for full processing gain. The spread spectrum nature of Mode 5 does, however, make those emissions less susceptible to noise produced by unrelated MIDS signals transmissions.
Accordingly, there is a need for a transponder receiver that can detect and process both wide and narrow band interrogation mode signals reliably in the presence of potential interference, wherein the processing of the wide band mode signals is not compromised by portions of the receiver that are used for processing of the narrow band mode signals.